A laser is a device which has the ability to produce monochromatic, coherent light through the stimulated emission of photons from atoms, molecules or ions of an active medium which have typically been excited from a ground state to a higher energy level by an input of energy. Such a device contains an optical cavity or resonator which is defined by highly reflecting surfaces which form a closed round trip path for light, and the active medium is contained within the optical cavity.
If a population inversion is created by excitation of the active medium, the spontaneous emission of a photon from an excited atom, molecule or ion undergoing transition to a lower energy state can stimulate the emission of photons of substantially identical energy from other excited atoms, molecules or ions. As a consequence, the initial photon creates a cascade of photons between the reflecting surfaces of the optical cavity which are of substantially identical energy and exactly in phase. A portion of this cascade of photons is then discharged out of the optical cavity, for example, by transmission through one or more of the reflecting surfaces of the cavity. These discharged photons constitute the laser output.
Excitation of the active medium of a laser can be accomplished by a variety of methods. However, the most common methods are optical pumping, use of an electrical discharge, and the passage of an electric current through the p-n junction of a semiconductor laser.
Semiconductor lasers contain a p-n junction which forms a diode, and this junction functions as the active medium of the laser. Such devices, which are also referred to as laser diodes, are typically constructed from materials such as gallium arsenide and aluminum gallium arsenide alloys. The efficiency of such lasers in converting electrical power to output radiation is relatively high and, for example, can be in excess of 40 percent.
The use of flashlamps, light-emitting diodes (as used herein, this term includes superluminescent diodes and superluminescent diode arrays) and laser diodes (as used herein, this term includes laser diode arrays) to optically pump or excite a solid lasant material is well-known. Lasant materials commonly used in such solid state lasers include crystalline or glassy host materials into which an active material, such as trivalent neodymium ions, is incorporated. Highly suitable solid lasant materials include substances wherein the active material is a stoichiometric component of the lasant material. Such stoichiometric materials include, for example, neodymium pentaphosphate, neodymium aluminum borate and lithium neodymium tetraphosphate. Detailed summaries of conventional solid lasant materials are set forth in the CRC Handbook of Laser Science and Technology, Vol. I, M. J. Weber, Ed., CRC Press, Inc., Boca Raton, Florida, 1982, pp. 72-135 and by A. A. Kaminskii in Laser Crystals, Vol. 14 of the Springer Series in Optical Sciences, D. L. MacAdam, Ed., Springer-Verlag, New York, N.Y., 1981. Conventional host materials for neodymium ions include glass, yttrium aluminum garnet (Y.sub.3 Al.sub.5 O.sub.12, referred to as YAG), YAlO.sub.3 referred to as YALO), LiYF.sub.4 (referred to as YLF), gadolinium gallium garnet (Gd.sub.3 Ga.sub.5 O.sub.12, referred to as GGG) and gadolinium scandium gallium garnet (Gd.sub.3 Sc.sub.2 Ga.sub.3 O.sub.12, referred to as GSGG). By way of example, when neodymium-doped YAG is employed as the lasant material in an optically-pumped solid state laser, it can be pumped by absorption of light having a wavelength of about 808 nm and can emit light having a wavelength of 1064 nm.
U.S. Pat. No. 3,624,545, issued to Ross on Nov. 30, 1971, describes an optically-pumped solid state laser composed of a YAG rod which is side-pumped by at least one semiconductor laser diode. Similarly, U.S. Pat. No. 3,753,145, issued to Chesler on Aug. 14, 1973, discloses the use of one or more light-emitting semiconductor diodes to end-pump a neodymium-doped YAG rod. The use of an array of pulsed laser diodes to end-pump a solid lasant material such as neodymium-doped YAG is described in U.S. Pat. No. 3,982,201, which was issued to Rosenkrantz et al. on Sept. 21, 1976. Finally, D. L. Sipes, Appl. Phys. Lett., Vol. 47, No. 2,1985, pp. 74-75, has reported that the use of a tightly focused semiconductor laser diode array to end pump a neodymium-doped YAG results in a high efficiency conversion of pumping radiation having a wavelength of 810 nm to output radiation having a wavelength of 1064 nm.
U.S. Pat. No. 4,847,851, issued to Dixon on July 11, 1989, discloses a diode-pumped solid state laser wherein the diode pump is butt-coupled to a laser gain material which absorbs 63% of the optical pumping radiation within a pathlength of less than 500 microns. In such a device, a divergent beam of optical pumping radiation from the diode pump is directed into a volume of the gain medium which has a sufficiently small transverse cross-sectional area so as to support only single transverse mode laser operation. In addition, J. J. Zayhowski et al., Optics Letters, Vol. 14, No. 1, pp. 24-26 (Jan. 1, 1989), have reported the construction of single-frequency microchip lasers which: (a) use a miniature, monolithic, flat-flat, solid-state cavity whose mode spacing is greater than the gain bandwidth of the gain medium; and (b) are longitudinally pumped with the close-coupled, unfocused output of a laser diode.
U.S. Pat. No. 4,173,738, issued to Boling et al. on Nov. 6, 1979, is directed to a solid state laser which has an output at two distinct wavelengths. This device is optically-pumped with white light and contains, as its active medium, an array of two different types of solid plate-like elements along a common optical axis. One type of element lases at one wavelength and the other at a second wavelength.